Apparatus for chemical mechanical polishing

ABSTRACT

A method and apparatus is disclosed for polishing the face of a semiconductor wafer. The wafer is held in position by a tooling head and is contacted by an abrasive pad. A table is provided on to which the abrasive pad is fixedly attached, both of which move in directions parallel to the face of the wafer being polished. A controller controls the motion of the table according to a predetermined polishing pattern and is capable of maintaining a constant velocity between the wafer and the abrasive pad. The tooling head includes a circular platen and a retention ring peripherally oriented about the outer edge of the platen which resists lateral forces on the wafer caused by engagement of the face of the wafer with the polishing surface. An adjustable coupling is mounted to the platen and the ring, and serves to adjustably position during polishing the height of the ring relative to the face of the wafer as well as to rigidly support during polishing the position of the retaining ring. A flexible disk is fixedly mounted between a support post and the platen and oriented substantially parallel to the face of the platen. The flexible disk is adapted to prevent rotation of the platen about the axis of the support post and to transmit forces between the platen and the post in directions parallel to the face of the platen.

This application is a division of application Ser. No. 08/443,956, filedMay 18, 1995, now pending.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to the polishing of semiconductor wafersof the type from which chips for integrated circuits and the like aremade. More specifically in a chemical mechanical process (CMP) asemiconductor wafer is held by a tooling head and is polished by contactwith an abrasive material in a controlled chemically active environment.

2. Discussion of the Related Art

As part of the manufacturing process of semiconductor devices,semiconductor wafers are polished by CMP. The uniform removal ofmaterial from and the planarity of patterned and un-patterned wafers iscritical to wafer process yield. Generally, the wafer to be polished ismounted on a tooling head which holds the wafer using a combination ofvacuum suction or other means to contact the rear side of the wafer anda retaining lip or ring around the edge of the wafer to keep the wafercentered on the tooling head. The front side of the wafer, the side tobe polished, is then contacted with an abrasive material such as anabrasive pad or abrasive strip. The abrasive pad or strip may have freeabrasive fluid sprayed on it, may have abrasive particles affixed to it,or may have abrasive particles sprinkled on it.

The ideal wafer polishing process can be described by Preston'sequation: R=K_(p) *P*V, where R is the removal rate; Kp is a function ofconsumables (abrasive pad roughness and elasticity, surface chemistryand abrasion effects, and contact area); P is the applied pressurebetween the wafer and the abrasive pad; and V is the relative velocitybetween the wafer and the abrasive pad. As a result, the ideal CMPprocess should have constant cutting velocity over the entire wafersurface, constant pressure between the abrasive pad and wafer, andconstant abrasive pad roughness, elasticity, area and abrasion effects.In addition, control over the temperature and Ph is critical and thedirection of the relative pad/wafer velocity should be randomlydistributed over the entire wafer surface.

Most current CMP machines fail to produce constant velocity distributionover the entire wafer surface which is necessary for uniform materialremoval and good flatness. One common type of wafer polishing apparatusis the CMP model 372M made by Westech Systems Inc. As shown insimplified form in FIG. 1, a wafer 100 is held by a tooling head 102which rotates about the axis of the wafer. A large circular abrasive pad104 is rotated while contacting the rotating wafer and tooling head. Therotating wafer contacts the larger rotating abrasive pad in an area awayfrom the center of the abrasive pad. Thus in the Westech apparatus, therelative motion between the wafer and the abrasive pad has twocomponents: one due to the rotating wafer and another due to therotating abrasive pad.

A number of disadvantages result from the relative motion between thewafer and the abrasive pad in the Westech apparatus. According toPreston's equation the rate at which material is removed from a givenpoint on the wafer is directly proportional to the relative velocitybetween that point and the abrasive pad. In the Westech apparatus,different points on the wafer see different relative abrasive padvelocities, and therefore have different removal rates. The non-uniformrelative velocities result from the fact that the velocity near thecenter of a rotating circle is less than the velocity near the outsideof the circle. For example, while the center of the rotating wafer seesa constant velocity which is related solely to the rotating abrasivepad, the outer points of the wafer see velocities which are acombination of the rotating tooling head and the rotating abrasive pad.

Not only does the Westech apparatus result in non-uniform velocities atdifferent points of the wafer at any one time, the velocities of pointsaway from the center tend to fluctuate significantly over time. Constantvelocities are preferable to fluctuating velocities since the removalrate and other factors necessary in obtaining a smooth finish are mucheasier to control. For example, with the Westech system, points awayfrom the center of the wafer see alternating high and low velocities.During a period of low velocity, the abrasive material may pit orscratch the surface of the wafer and result in a non-smooth surface.

Another related apparatus is a polishing machine for polishingsemiconductor wafers containing magnetic read-write heads, disclosed inU.S. Pat. No. 5,335,453 to Baldy et al. With this machine, asemiconductor wafer is held by a support head which is moved in acircular translatory motion by an eccentric arm. The wafer is polishedby contacting an abrasive strip which is advanced in one direction. Therelative motion between the wafer and the abrasive strip is acombination of the circular motion of the wafer and the linear motion ofthe advancing abrasive strip. The resulting relative motion is that of acircle precessing in a straight line. Note that "precessing" used hereinrefers to the movement in a plane of an axis which is perpendicular tothe plane and about which a circular translation occurs.

While the precessing circle polishing pattern that this apparatusprovides should provide more uniform velocities such that differentpoints on the wafer see similar velocities at any given time, thevelocities are still not constant. Assuming the rotation of theeccentric arm is held to a constant angular speed, the precessing circlerelative motion results in fluctuating velocities. When the wafer isrotating away from the precessing direction the net relative velocity islower, and when the wafer is rotating with precessing direction the netrelative velocity is higher.

Moreover, the apparatus has the disadvantage of not being able toprovide alternative polishing patterns. Since the support head ismounted on a rotating eccentric arm, the wafer can only be polished bymoving in a circle. Polishing patterns other than circular are desiredfor a number of reasons.

One such reason is to provide more uniform wear of the abrasive pad.Non-uniform wear of the abrasive pad results in a non-uniform removalrate of wafer material since more heavily worn sections of the abrasivepad remove material at a lower rate. Non-uniform wear also results inless efficient use of the abrasive pad itself, since the pad must bechanged more often or advanced at a faster rate in order to avoid usingportions of the pad which wear out first.

A relative polishing motion of a circle precessing in a straight lineresults in the center of the abrasive strip having more wear than theoutside edges. This is because the wafer spends more time in the centerof the pad than at the outer edges. The more time the wafer spends oncertain sections of the abrasive strip, the more the abrasive stripwears out.

Another disadvantage of circular patterns is uncontrolled spinning ofthe wafer inside the tooling head. The forces from the polishing motionin a continuous circular motion cause the wafer to tend to spin orrotate in one direction with respect to the tooling head.

An additional disadvantage of only providing a circular pattern is thatcertain topologies on the wafer surface may be less well suited forcircular polishing patterns. The pre-polishing topology of the surfaceof the wafer may be patterned, due to processing. Each surface topologyis optimally planarized by a certain polishing pattern, which may notalways be a circular one. Thus, providing the ability to custom designpolishing patterns is desirable so that polishing different surfacetopologies may be optimized.

Another disadvantage with prior systems is that it is difficult orimpossible to polish selected regions of the wafer using a specificportion of an abrasive pad. For example, if a zone of aggressiveabrasive is present on the abrasive pad, it is difficult to selectivelyuse that zone to increase the removal rate on certain parts of thewafer, if the only motion provided by the polishing system is that of afixed radius circle.

Another disadvantage with prior systems is related to the fact that awafer moving in a particular direction will have a higher removal rateon the leading edge and sides of the wafer. Prior art systems providingonly certain polishing patterns cannot be easily made to control theremoval rates at the edges due to this effect. Circular motion generallyprovides a constantly variable cutting direction and therefore willcause removal rates from all the edges. However, prior art systemscannot be programmed so that certain areas are selectively polished byspending more time traveling in one direction than another.

While Preston's equation, explained above, describes the ideal process,it does not address some factors related to a manner in which the waferis presented to the polishing media and chemistry. Many CMP machinescurrently available yield wafers bearing anomalies in planarity. Theseanomalies are often related to factors that cannot be described byPreston's equation. For example, it has been found with many availableCMP machines, that the removal rates of material are higher near theedges of the wafer. The shape of the platen (which holds the waferagainst the polishing media) and the relationship between the platen andthe retaining ring (which keeps the wafer centered on the platen) arecrucial to the final wafer planarity.

One attempt at addressing some of the problems not described byPreston's equation is described in U.S. Pat. No. 5,205,082 to Shendon etal. Disclosed is an attempt to control the relationship between theplaten, ring and pad by tying the platen and the ring together through aflexible diaphragm. However, by allowing the ring to float with respectto the platen, the ring can be upset by changes in abrasive padflatness, roughness, and friction. When the ring is disturbed, thepressure on the periphery of the wafer increases. This can contribute topoor planarity because of more pronounced oxide removal rates near thewafer edges.

Another type of tooling head, described in U.S. Pat. No. 4,954,142 toCarr et al., uses a retaining ring with constant pressure at all pointson the ring. The pressure on the ring is not adjustable duringpolishing, in that different springs must be used for any variation inconditions, such as changes in the abrasive pad surface.

Changing springs requires disassembly of the retention ring from thehead. Thus there is a need for a tooling head having a retaining ring inwhich the pressure between the ring and the abrasive pad is easilyadjustable, and less prone to changes in the abrasive pad surface.

Other problems with tooling heads include undesirable chattering orvibration during certain polishing patterns. Chattering or vibration iscaused by allowing a certain degree of backlash, or play between thetooling head and its supporting member in directions parallel to thewafer being polished.

SUMMARY OF THE INVENTION

Thus it is an object of the present invention to provide an apparatuswhich polishes wafers while maintaining uniform relative velocity at allpoints on the wafer relative to the abrasive pad.

It is also an object of the present invention to provide an apparatuswhich polishes wafers while maintaining uniform average velocity betweenthe wafer and the abrasive pad.

It is a further object of the invention to provide an apparatus whichpolishes wafers while providing uniform wear of the abrasive pad.

It is a further object of the invention to provide an apparatus whichpolishes wafers while controlling the rotation of the wafer inside thetooling head, thus minimizing deformation or breakage of the wafers.

It is a further object of the invention to provide an apparatus whichpolishes wafers by offering the user alternative polishing patterns.

It is a further object of the invention to provide a more effectivetooling head which has the capability of reducing problems of poorplanarity.

It is a further object of the invention to provide a more effectivetooling head which does not allow backlash and thus prevents undesirablechattering or vibration during polishing.

In order to meet these objectives, the present invention is directed toan apparatus for polishing a semiconductor wafer which includes a tablehaving a polishing medium fixedly attached to its surface and beingmovable in directions parallel to the surface of the table. A controlleris provided for controlling the motion of the table according to apredetermined polishing pattern. The polishing pattern preferablymaintains a constant velocity between the wafer and the abrasive medium,and consists of a series of arc shaped paths.

The wafer is held in place and in contact with the abrasive pad by atooling head which includes a circular platen and a retention ringperipherally oriented about the outer edge of the platen. The retentionring is mounted and positioned to resist lateral forces on the wafercaused by engagement of the face of the wafer with the polishingsurface. An adjustable coupling is mounted to the platen and the ring,and serves to adjustably position during polishing the height of thering relative to the face of the wafer as well as to rigidly supportduring polishing the position of the retaining ring.

A flexible disk is fixedly mounted between a support post and the platenand oriented substantially parallel to the face of the platen. Theflexible disk is adapted to prevent rotation of the platen about theaxis of the support post and to transmit forces between the platen andthe post in directions parallel to the face of the platen.

Additionally, a cavity is defined by the platen, wall member, and topplate. A port and a pressure regulator are used to introduce gas orliquid into the cavity at positive or negative pressures. The pressurein the cavity causes the platen to deform, thus deforming the wafer toaffect the polishing characteristics as desired by the user.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified prospective view of a prior art CMP system;

FIG. 2 is a front prospective view of a polishing apparatus according tothe present invention;

FIG. 3 is a front prospective view of a tooling head, an abrasive pad,and a table according to the present invention;

FIG. 4 is a block diagram illustrating control circuitry according to apreferred embodiment of the present invention;

FIG. 5 is a top view of a polishing pattern according to the presentinvention;

FIG. 6 is a front prospective view of a polishing apparatus with aneccentric arm, according to an alternative embodiment of the presentinvention;

FIGS. 7a-b are front prospective views of non-rotation means of apolishing apparatus with an eccentric arm, according to the presentinvention;

FIG. 8 is a front prospective view of another embodiment of a polishingapparatus according to the present invention;

FIG. 9 is a side view cross-section of a tooling head, according to thepresent invention;

FIG. 10 is a sectional view of a tooling head according to anotherembodiment of the invention;

FIG. 11 is a partial sectional view of a tooling head, illustrating thearea near the retention ring, according to the embodiment of FIG. 10;

FIG. 12 is a partial sectional view of a tooling head, illustrating thearea near the retention ring, according to the present invention;

FIG. 13 is a front prospective view of a tooling head, illustrating theX and Y axes of rotation, according to the present invention;

FIG. 14 is a block diagram illustrating control circuitry forcontrolling the retention ring according to a preferred embodiment ofthe present invention;

FIGS. 15a-b are side view cross-sections of a tooling head, illustratingthe diaphragm action, according to the present invention; and

FIGS. 16a-f are side view cross-sections of the tooling head,illustrating the use of varying platen thickness and multiple chambers,according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The following embodiments of the present invention will be described inthe context of a method and apparatus for polishing semiconductorwafers, although those skilled in the art will recognize that thedisclosed methods and structures are readily adaptable for broaderapplication, e.g., polishing of other substrates. For example, theinvention is readily adaptable for use in processing other types of diskshaped objects. Note that whenever the same reference numeral isrepeated with respect to different figures, it refers to thecorresponding structure in each such figure.

A method and apparatus for polishing semiconductor wafers according tothe present invention is illustrated in FIG. 2. The rear side of wafer200 is held by tooling head or wafer carrier 202, while the front sideof wafer 200 is contacted by abrasive pad 206. Wafer carrier 202 isconnected to a post 204 which may move tooling head or wafer carrier 202and wafer 200 in the Z-direction, which is perpendicular to the plane ofwafer 200, so that wafer 200 may be brought into contact with abrasivepad 206. Post 204 may also apply polishing force in the Z-direction onor wafer carrier 202 and wafer 200. The Z-direction movement and forceapplied to tooling head 202 is provided by a servo. In this embodimentof the invention, the servo includes a lead screw 212, which pushes aplate 214 attached to a linear slide 216. Cross-member 218 is fastenedto plate 214 and also to post 204. According to a preferred embodimentof the invention lead screw 212 is driven by an electric motor 213mounted to base 229 and which is computer controlled so that the usermay program the force applied during the polishing process. One skilledin the art will realize that other methods providing Z-directionmovement and force are practicable.

Post 204 and tooling head 202 hold wafer 200 in a substantially fixedposition in X and Y-directions, which are parallel to the plane of wafer200 and perpendicular to each other. According to this embodiment of theinvention, tooling head 202 and wafer 200 do not rotate about an axisperpendicular to and passing through the center of wafer 200.

Table 208 is movable in both the X and Y-directions. According to apreferred embodiment of the invention, table 208 is movable in theX-direction by action of lead screw 220 and linear slide 222. Similarlytable 208 is movable in the Y-direction by being mounted on plate 224which is mounted to linear slide 228 and actuated by lead screw 226.Note that linear slide 228 is also mounted to base 229. In a preferredembodiment of the invention, lead screws 220 and 226 are driven byinfinitely positionable electric motors 221 and 227 which are mounted toplate 224 and base 229 using brackets 223 and 227 respectively. Motors221 and 227 are preferably computer controlled so that the user mayprogram the table to move in an infinite number of patterns.

While lead screws are used in the presently preferred embodiment of theinvention, one skilled in the art would recognize that other servo meanswould be practicable, for example a rack-and-pinion servo means.

Movements of table 208 in the X and Y-directions while holding wafer 200and tooling head 202 fixed in the X and Y-directions result in relativemotion between the wafer 200 and abrasive pad 206. Shown in FIG. 3 is anexample of a polishing pattern 230, which is in this case a series oftangent circles. The pattern shows a path traced by the center of wafer200 as the abrasive pad 206 is moved beneath it.

Note that since wafer 200 does not rotate with respect to abrasive pad206, all points on the wafer experience the same relative velocity withrespect to the abrasive pad.

Providing uniform relative velocity for all points on the waferadvantageously provides a more uniform removal rate of material from thewafer when compared to prior art systems where the wafer rotates.

In the system described according to the present invention, the user mayprogram the movement of the abrasive pad and table such that therelative velocity of the wafer with respect to the abrasive pad isconstant at all times. Constant velocity of the wafer is desirable sinceit results in more predictable material removal rates than non-constantvelocity polishing patterns.

According to a preferred embodiment of the invention, force between thewafer and the abrasive pad is controlled by compression springs whichreside in tooling head 202 (shown in FIG. 9). Since the tooling head ismoveable in the Z-direction, under computer control, the user is free toprogram the force applied between the wafer and abrasive pad in order toimprove slurry or cutting fluid distribution and cutting efficiency. Forexample, a variable force applied during polishing may in some casesimprove polishing results.

Note that since abrasive pad 206 is fixedly attached to table 208, ahigher force may be applied between the wafer and pad than is possiblewith prior art systems using an advancing movable abrasive tape. Byfixing the abrasive pad to the table, the force of the head pushing downon the tape in the Z-direction helps retain the tape in place. Incontrast, with systems using advancing tape abrasive the force of thehead pushing down on the tape must be overcome in order to advance thetape. Thus with advancing tape systems a lower force must be applied inthe Z-direction to maintain a flat tape surface and to prevent tearing.Moreover, with advancing tape systems, the abrasive tape has a greatertendency to stretch, pucker or deform under the great forces subjectedto it during operation. Such deformations and stretching cause lessconsistency in the velocity of the tape speed, and, as a result, lessconsistent relative velocity between the tape and wafer.

Since the system according to the present embodiment uses an abrasivepad affixed tm the table, and the table is moved by infinitelypositionable motors with a lead screw drive, very accurate andconsistent velocity can advantageously be achieved.

FIG. 4 is a block diagram illustrating in simplified form the controlcircuitry according to a preferred embodiment of the invention.Polishing pattern data memory 330 contains position information of table206 in the X and Y-directions, and tooling head 202 and post 204 in theZ-direction according to a predetermined polishing pattern. The patterndata is received by the X, Y, and Z position controllers 332, 334, and336. The position controllers compute the change in position requiredusing both the position data from memory 330 and feedback informationfrom the electronic motors. The change in position data is sent to motordrivers 338, 340, and 342, which use the received information to drivemotors 227, 221 and 213. Preferably position encoders are provided onthe motors or elsewhere which send position feedback to the positioncontrollers. One of ordinary skill in the art may select the componentsneeded for the control circuitry from many which are commerciallyavailable.

If uniform removal of material is desired, care must be taken inprogramming a particular polishing pattern so that the cut angle andleading edge of the wafer are sufficiently varied. The cut angle refersto the direction of relative movement that a point on the wafer seeswith respect to the abrasive pad. If the cut angle is not sufficientlyvaried, grooves may be cut in the surface of the wafer.

Similarly, the leading edge of the wafer should be sufficiently variedduring the polishing process, since the leading edge of the waferexperiences a higher rate of material removal. If the leading edge ofthe wafer is not varied sufficiently, non-uniform removal rates fromdifferent parts of the wafer may result. It has been found that circularand arc polishing patterns provide sufficient variations in both cutangles and leading edges of the wafer.

Thus, according to this embodiment of the present invention, a waferpolishing machine is provided which allows the user to program anypolishing pattern desired. The user is not confined to a fixed polishingpattern, such as a fixed radius circle precessing in a straight line asin prior art systems. With the current system, for example, the user mayprogram the pattern to be a circle with a different radius. Rather thanprecessing in a straight line the user may choose a sine wave, a largercircle, or any combination of arcs, curves, or straight lines. The usermay also program any of the precessing patterns to move in a "figure 8"pattern instead of a circle.

The advantages of providing a wide variety of programmable polishingpattern are numerous. As discussed above, the user may program therelative wafer to pad velocity to be constant, as opposed to priorsystems where the velocity fluctuates. The user may also program thepattern such that detrimental effects of spinning or rotating of thewafer with respect to the tooling head is minimized. For example, theuser may program a circular polishing pattern in which the direction ofrotation is reversed at intervals such that the wafer does not spin inthe tooling head significantly in one direction.

Another advantage of providing a wide variety of alternative polishingpatterns is that particular topologies on the surface of the wafer maybe better planarized by selecting a more optimal polishing pattern.

Another advantage of providing a wide variety of alternative polishingpatterns is the ability to take advantage of regions of the abrasive padwhich have a more aggressive abrasive. The user may program the patternsuch that selected portions of the wafer are polished by the moreaggressive abrasive regions in order to increase removal rate from thoseportions. Similarly, if certain regions of the abrasive pad are lessabrasive, the user may program the polishing pattern such that the lessabrasive region is either avoided, or passed over uniformly so that aneven removal rate is still obtained.

Another advantage of the described system is that the user may programthe polishing pattern to selectively remove more material from certainedges of the wafer by programming the pattern such that those edges aremore frequently leading in the polishing pattern.

Another advantage is that the user may program the polishing patternsuch that uniform wear of the abrasive pad is achieved. According to apreferred embodiment uniform abrasive pad wear has been attained bypolishing in a pattern of continuous arcs which together form a seriesof tangent circles.

FIG. 5 shows an example of one continuous arc polishing pattern. Thecontinuous arc pattern shown comprises a series of arcs traced out bythe center of a wafer being polished such that a series of tangentcircles is drawn. Paths 1 to 5 are shown with different lines so thatthe reader may more easily follow the path of the wafer on the abrasivepad. In practice, the center of the wafer moves continuously from onepath to the next, while maintaining a constant velocity during thepolishing process.

The continuous arc pattern shown in FIG. 5 has been found to provide anextremely good polishing result for a number of reasons. Pad wear isuniform, since the wafer covers a large area in a uniform manner priorto repeating the pattern. The cut angle and leading edge of the wafer iscontinuously varied since the wafer is continuously in an arc motion.Uniform velocity is attained at all points on the wafer since the waferdoes not rotate. Constant velocity can be maintained throughout thepolishing pattern, so that a more uniform removal rate of material isattained. Finally, spinning of the wafer against the tooling head causedby continuous circular motion in one direction is minimized, since thearcs in the pattern are alternated between clockwise andcounterclockwise rotational directions.

FIG. 6 shows another embodiment of the present invention. In thisvariation, tooling head or wafer carrier 202 is connected to crossmember 218 by eccentric arm 232. Eccentric arm 232 is rotatable about anaxis "A" which is concentric with the upper portion of eccentric arm232, so that tooling head 202 is moved in a circular path. Note that asin the previous embodiment abrasive pad 206 and table 208 are movable inthe X and Y-directions by lead screws 220 and 226, and motion and forceapplied in the Z-direction are provided by lead screw 212.

According to this embodiment of the invention, the rotation of eccentricarm 232 is computer controlled, so that the user may precisely controlthe rotation speed. By rotating eccentric arm 232, tooling head 202 andwafer 200 are moved in a circular path about axis "A." If table 208 isalso moved in the X and Y-directions, the resulting polishing pattern isthat of a precessing circle. Unlike prior art systems which provide onlystraight-line precessing circles, this embodiment of the inventionprovides the user with unlimited precessing circle patterns. Forexample, the circular pattern may be made to precess in a larger circleor a sine wave pattern.

Preferably, tooling head 202 does not rotate about its own central axis"B." Instead tooling head 202 and wafer 200 translate in a circular pathabout axis "A." This translation without rotation results in uniformrelative velocities attained at all points of the wafer. FIGS. 7a-b showtwo alternative embodiments of eccentric arm 232 and tooling head 202which include structures to ensure that tooling head 202 does not rotatewhile tracking a circular path.

The structure shown in FIG. 7a uses a pulley and belt system to ensurenon-rotation. Movement along the circular path is provided by a rotatingvertical post 236, which rotates inside a non-rotating fixed verticalpost 234 and protrudes as shown. Eccentric cross member 238 is connectedto rotating vertical post 236 and spindle 240. Fixed pulley 242 isfastened to fixed vertical post 234. Belt 243 is engaged to both fixedpulley 242 and head pulley 244. Head pulley 244, in turn, is fastened tohead post 246 which passes through and is supported by spindle 240 andis fastened to tooling head 202.

In operation, rotating vertical post 236 spins and causes eccentric arm238, spindle 240, head post 246 and tooling head 202 to move in acircular path. Cooperation between fixed pulley 242, belt 243 and headpulley 244 ensures that tooling head 202 translates rather than rotatesas it moves through its circular path.

FIG. 7b show another embodiment of eccentric arm 232 which uses a seriesof gears to ensure non-rotation of tooling head 202. As shown, fixedvertical post 234, rotating vertical post 236 and eccentric cross member238 are provided in a fashion similar to the previous embodiment. Inthis embodiment, fixed gear 248 is fastened to fixed vertical post 234and engages intermediate gear 250, which, in turn, engages head gear252. Head gear 252 is fastened to head post 246. In operation, eccentriccross member 238 rotates about axis of rotation "A," while the series ofgears 248, 250, and 252 ensure that neither fixed post 246 nor toolinghead 202 rotate, but rather translate as they move in a circular motionabout axis "A." Persons of ordinary skill in the art may select theappropriate gears or pulleys based on the length of arm 232 to achievethe effect as described herein.

Note that if a constant, non-fluctuating velocity between the wafer andthe abrasive pad is desired, the rotation speed of eccentric arm 232 maybe programmed accordingly. In order to compensate for the relative x andY-direction motion of the table 208, the rotation speed of eccentric arm232 may be programmed to keep the linear speed between the wafer and theabrasive pad at a constant value. Preferably, the eccentric arm shouldrotate at a slower speed when the motion of the head has a componentwhich is opposite in direction to the table motion component and at afaster speed when the head motion has a component in the same directionas that of the table.

FIG. 8 is a perspective view of another alternative embodiment accordingto the invention. In this embodiment abrasive pad 206 is mounted totable 208' which moves in the Y-direction by action of linear slide 228'and lead screw 226'. Wafer 200 is held by head 202 which is mounted oneccentric arm 232. Eccentric arm 232 is rotatably mounted tocross-member 218 which is fastened to plate 214. Under action of leadscrew 212, plate 214 moves in the Z-direction. In this embodiment,linear slide 216 is slidably mounted to plate 318 which is fixedlymounted to plate 320. Plate 320 moves in the X-direction under action oflead screw 220' and linear slide 222'. Thus in this embodiment, head202, arm 232, and cross-member 218 move in both the X and Z-directions,while the table 208' moves in the Y-direction. One skilled in the artwould recognize that other embodiments are possible. For example, thetable may remain stationary in the X and Y-directions, while the headmoves in the X and Y-directions.

FIG. 9 is a cross-section side view of tooling head or wafer carrier202, according to one embodiment of the invention. As shown, post 204 isconnected to shoulder 260 and provides motion and force in theZ-direction. Shoulder 260 is a solid cylindrical piece, and has acylindrical guide ball socket 262 at its center which mates with guideball 264. Guide ball 264 is securely fastened to platen 266 with a boltor the like. As shown, wafer 200 is contacted by the underside of platen266. Retaining lip 273 protrudes below the surface of platen 266 andserves to keep the wafer centered on the platen by contacting the edgesof the wafer. Guide ball 264 and guide ball socket 262 provide accuratepositioning of platen 266 and shoulder 260 in the X and Y-directions.

Preferably guide ball 264 fits tightly into guide ball socket 262 suchthat guide ball 264 allows less than about 0.0002inch of movement in theX and Y-directions.

Shoulder 260 also contains a circular notch 268 into which fitcompression springs 270, which act between shoulder 260 and platen 266.In a preferred embodiment, springs 270 comprise a stack of circularBellville springs that rest on platen 266. Springs 270 ensure thatplaten 266 is biased against the wafer and table so that the force inthe Z-direction and thus pressure on the wafer can be varied. Thereforeby varying the position of shoulder 260 and post 204 in the Z-direction,the pressure between wafer 200 and the abrasive pad may be accuratelycontrolled.

Also shown in FIG. 9 is flexure ring 272, preferably a thin circularsteel ring which is mounted to both the perimeter of shoulder 260 and araised edge portion 267 of platen 266 using annular clamp rings 269 and271 which are bolted to raised portion 267 and shoulder 260respectively. Flexure ring 272 transmits cutting forces and torquebetween shoulder 260 and platen 266 in the X and Y-directions. Flexurering 272 also allows relative motion in the Z-direction so that forcecan be transmitted to the wafer, and allows platen 266 to pivot slightlyin the Z-direction about guide ball 264, as shown, in order to complywith slight variations in pad angle. However, flexure ring 272 does notallow rotation about the Z-axis. Advantageously, flexure ring 272 andguide ball 264 prevent platen 266 from moving in the X and Y-directions,so that there is no backlash, which can lead to undesirable chatteringor vibration when using some polishing patterns.

FIG. 10 is a sectional view of a tooling head according to anotherembodiment of the invention. Wafer 200 is shown being held by toolinghead 202. The rear side of wafer 200 is contacted by resilient pad 298,which preferably comprises an elastomer material, and acts as a springbetween flexible platen 277 and wafer 200 and is compressible underforce in the Z-direction. Flexible platen 277 comprises a thicker wallportion 276 and a thinner face portion 279. Platen 277 preferablyconsists of a metal material such as steel. The wall portion of platen277 is attached to top plate 274, and cavity 278 is defined by the walland face portions of platen 277 and top plate 274. Cavity 278 may befilled with gas or liquid to exert a positive or negative pressure onthe face portion of platen 277 which acts as a diaphragm. Port 280 isprovided to supply and drain gas or liquid from cavity 278. According toa preferred embodiment, an electronic computer controlled pressureregulator 281 is provided so that the pressure in cavity 278 isprogrammable and accurately controlled. By controlling the amount offluid or gas in cavity 278, the face of platen 277 can be caused to flexinto a concave or convex shape, thus advantageously changing the shapeof the wafer 200 during polishing. Wall 276 and platen 277 join to forma "living hinge" or notch 277a which functions to allow the face ofplaten 277 to minimize geometric distortions at the periphery andclosely assume a substantially spherical or continuous arc shape. Livinghinge 277a is formed circumferentially about the platen 277. By allowingadditional flexure at the junction between the wall 276 and platen 277,living hinge 277a prevents distortions which would otherwise occur inthe outer periphery of the surface 112aupon flexure or crowning of thesurface 112a, if so such living hinge were present. These advantages aremore clearly shown in FIGS. 15a and 15b.

FIG. 11 is a partial sectional view of a tooling head illustrating thearea near the edge of the wafer, according to the embodiment describedwith respect to FIG. 10. As shown in FIG. 11, Wafer 200 is held in placeby retention ring 282, which includes wear ring 291, and surrounds theperimeter of wafer 200. Note that retention ring 282 and plastic wearring 291 have curved or cambered outer edges, which aid in flatteningthe abrasive pad during polishing, advantageously resulting in moreuniform material removal from the wafer. Preferably, wear ring 291comprises a plastic material having both high abrasion resistance toresist abrasion by the 35 abrasive pad, and good dimensional stabilityto resist warpage. Retention ring 282 is flexibly attached to top plate274 by retention ring flexure member 284. Retention ring flexure 284comprises a flat circular steel band and is fastened to top plate 274with clamp ring 275 which is bolted to top plate 274 (shown in FIG. 10).Flexure member 284 allows retention ring 282 to move in a verticaldirection, which is substantially perpendicular to the plane of wafer200. The exact vertical position of retention ring 282 is controlled bya number of servo-operated screws 288 and sensors 290, which are spacedalong the perimeter of the tooling head. Sensors 290 measure thevertical height of retention ring 282 relative to top plate 274, andpreferably send the information to a computer controller. The computeralso controls the servo-operated screws 288, which set the verticalposition of retention ring 282 during operation.

While the vertical position of retention ring 282 is adjustable, thescrews 288 and flexure member 284 rigidly hold the ring in place andthus cannot be disturbed by variations in the process materials, such as"waves" and the like in abrasive pad 206. Retention ring 282 flattensout these variations which results in a more predictable and stablepolishing process.

Table mounted sensor 292 is provided to measure the relative positionsof wafer 200 and wear ring 291 during polishing. Table mounted sensor292 comprises measuring pin 286, rubber diaphragm 293, sensor cavity294, gas port 295, transducer 296, liquid port 297 and sensor housing301. Measuring pin 286 has a rounded upper portion which protrudes abovethe surface of table 208 and into the region of abrasive pad 206. Whenthe retention ring 282 and wafer 200 pass above table mounted sensor292, measuring pin 286 comes in contact with and is depressed downward.Measuring pin 286 is movably linked to transducer 296 by threaded shaft299. Transducer 296 is a linear voltage displacement transducer whichtransforms the linear motion of shaft 299 into a voltage analog signal.

Sensor cavity 294 is defined by the walls of sensor housing 301, thetransducer, measuring pin, and rubber diaphragm. Gas port 295 providesmeans to control the gas pressure in sensor cavity 294. A positivepressure in cavity 294 serves to exert pressure on rubber diaphragm 293causing the diaphragm to push measuring pin 286 upward. Thus, thevertical position of and upward spring force on measuring pin 286 may becontrolled by the pressure of gas in the sensor cavity. Rubber diaphragm293 also serves to seal sensor cavity 294 from the liquid environment onthe surface of table 208. Liquid, such as water is pumped through liquidport 297, though the passageway and into area above the rubberdiaphragm. During operation, the liquid from port 297 flushes this upperarea of and generally keeps the area free from abrasive slurry.

Referring now to FIG. 12, a close-up cross section of the area nearretention ring 282 is shown, including wear ring 291, flexible platen277, resilient pad 298, wafer 200, abrasive pad 206 and table 208. Notethat flexure member 284 and other structures are not shown. Also shownis height relationship H which is the difference in the verticaldirection between wafer 200 and wear ring 291. During polishing, theheight relationship H is an important parameter in ensuring even removalof material. Small changes in this relationship can have a measurableeffect on uniformity. Even normal variations in wafer thickness if notcompensated for can make process planarity difficult to control.

As mentioned, the vertical position of retention ring 282 is accuratelycontrolled by manual operated or servo-operated screws. By controllingthe height of retention ring 282 according to measurements taken by ringposition sensors 290 and table mounted sensor 292, the heightrelationship H is fully adjustable. Advantageously, table mounted sensor292 is capable of sensing the relative height of wafer 200 and retentionring 282 under loaded conditions before or during polishing. Bymeasuring the relative position of the ring and the wafer under loadedconditions, factors such as the amount of compression of resilient pad298 and abrasive pad 206 are taken into account.

Being able to adjust the relationship H between the height of wafer 200and retention ring 282 advantageously allows control over thedistribution of force applied between wafer 200 and wear ring 291. Forexample by increasing the height relationship H, more force will beexerted in the Z-direction on wafer 200 and less force will be exertedon wear ring 291. Note that when force is applied, resilient pad 298acts as spring. By adjusting the height relationship H, the compressionof resilient pad 298 is changed which in turn changes the distributionof pressure between the wafer and the retention ring.

According to a preferred embodiment of the invention, Retention ring 282is also slightly rotatable about axes parallel to the plane of thewafer. This slight rotation is illustrated in FIG. 13, which is a muchsimplified view of wafer 200 and retention ring 282, not drawn to scale.FIG. 13 illustrates the relative rotational motion of retention ring 282about two axes X and Y, both axes being parallel to wafer 200. Forillustrative purposes, the relative position of retention ring 282 isgreatly exaggerated. Note that in the present embodiment of theinvention, servo operated screws 288 (shown in FIG. 11) are adjusted toposition retention ring 282 as desired. Note further, that flexuremember 284 allows retention ring 282 to rotate slightly about axes X andY, and translate vertically in the Z-direction.

FIG. 14 is a block diagram illustrating control circuitry used tocontrol the retention ring according to a preferred embodiment of theinvention. Controller 350 correlates readings from the ring positionsensors 290 of offsets in the Z-direction, and rotations of the ringabout the X and Y axes. Controller 350 compares the desired retentionring position from memory 330 with the readings from ring positionsensor and determines what if any change of position is required. Thecontroller computes which screws must by actuated to attain the desiredring position. Controller 350 then sends control information reflectingany adjustments necessary to drivers 352 which drive servo-operatedscrews 288. In this way, the vertical position of the ring as well asthe rotational position of the ring about the X and Y axes are preciselycontrolled and fully programmable. One of ordinary skill in the art mayselect the components needed for the described control circuitry fromthose currently commercially available.

By maintaining precise control over the position of the retention ring,the present invention provides the advantage of being able to fine tunethe pressure distribution on the wafer in order to achieve a moreuniform removal rate and surface finish on the wafer.

Referring now to FIGS. 15a-b, a more detailed description of the use ofcavity 278 to change the contour of wafer 200 will be given. Note thatthe contours shown in FIGS. 15a-b are exaggerated to illustrate therelative shapes. As mentioned, gas or fluid may be supplied via port 280which is preferably under control of a computer controlled electronicpressure regulator. As shown in FIG. 15a, when pressure builds in cavity278, flexible platen 277 is forced into a convex position as shown,where the center portion of wafer 200 is lower than the edge portions.In contrast, FIG. 15b shows a concave contour which is caused by avacuum or low pressure in cavity 278. By varying the pressure in cavity278 using port 280 and the pressure regulator, the contour of platen 277and wafer 200 may be advantageously programmed to range from highlyconvex to highly concave.

In FIGS. 16a-f, various embodiments of platen 277 and top plate 274,which allow for various contour shapes, are diagrammaticallyillustrated. In FIGS. 16a-d, the thickness of flexible platen 277 isvaried to change the contour shape. In general, thicker portions ofplaten 277 will exhibit a lower curvature than thinner portions underthe same pressure. In FIGS. 16a-b platen 277 comprises a thick centerportion 306. FIG. 16a shows a flat contour due to ambient pressure incavity 278, while FIG. 16b shows an convex contour due to high pressurein cavity 278. As shown in FIG. 16b, as a result of thick center portion306, the center portion of platen 277 has relatively little curvaturecompared to regions away from the center which have greater curvatures.Shown in FIG. 16c-d, platen 277 comprises a thick portion 308 which isring shaped. FIG. 16c shows a flat contour due to ambient pressure incavity 278, while FIG. 16d shows, as a result of high pressure, zones oflow curvature near thick portion 308 and zones of high curvature inother areas.

In FIGS. 16e-f, an alternative embodiment is shown wherein cavity 278 isdivided into multiple chambers by membrane member 370 which forms apressure tight seal between top plate 274 and platen 277. Thus, toolinghead 202 comprises a center chamber 310 and a perimeter chamber 312.Each chamber has a separate port, see ports 314 and 316, and isseparately pressurized or evacuated with fluid or gas. Shown in FIG. 16fis a contour resulting from a vacuum in center chamber 310 and highpressure in perimeter chamber 312. In this state, the center portion ofplaten 277 is concave while the outer portion is convex. One skilled inthe art will recognize that other contour patterns are possible usingdifferent relative pressures in the chambers or different chambersaltogether. For example the flexible platen 277 may be convex when thereis no pressure in the cavity 278 and less convex when there is a vacuumin cavity 278.

Thus by providing a platen having non-uniform thickness, or providingmultiple cavities or chambers, according to the present invention, auser may advantageously change the contour of a wafer being polished,and the removal rate may advantageously be more precisely controlled.

Note that although the two features just described, the operation andcontrol of retention ring 282, and the operation and control ofdeformable platen 277 and cavity 278, have been described in conjunctionwith one another, one of ordinary skill in the art will recognize thateither one may be practiced independently of the other. In other words,the adjustable retention ring will operate with a conventional platen,and a deformable platen will operate with a conventional retaining lip.

The embodiments of this invention described above are illustrative onlyof the principles of this invention and are not intended to limit theinvention to the embodiments described herein. In view of thisdisclosure, those skilled in the art can utilize this invention in awide variety of applications. For example, one skilled in the art willrecognize that the present invention may be used for polishing otherdisk-shaped objects.

We claim:
 1. An apparatus for polishing a substrate comprising:a platenhaving a face adapted to contact the substrate during polishing, saidface of said platen being substantially planar; a post member movablyattached to said platen, said post member having an axis substantiallyperpendicular to said face of said platen; a flexible disk fixedlymounted to said post member and to said platen, said flexible diskoriented substantially parallel to said face of said platen, dimensionedto prevent rotation of said platen about said axis of said post memberand to transmit forces between said platen and said post member indirections parallel to said face of said platen; and at least onebiasing member between said post member and said platen, said biasingmember biasing said post member and said platen away from one another.2. The apparatus of claim 1, further comprising:a guide ball fixedlymounted to said platen and movably connected to said post member, saidguide ball positioned to allow said platen to tilt with respect to saidpost member along axes parallel to said face of said platen and passingthrough the center of said guide ball.
 3. The apparatus of claim 1,further comprising:a table having an upper surface; and a polishingmedium attached to said upper surface of said table; wherein said platenis adapted to force the substrate against said polishing medium duringpolishing.
 4. An apparatus for polishing a substrate, comprising:aplaten having a face adapted to contact the substrate during polishing,said face of said platen being substantially planar; a post memberhaving an axis substantially perpendicular to said face of said platenand configured for applying force to said platen in a direction alongsaid substantially perpendicular axis; a flexible member fixedly mountedbetween said post member and said platen, said flexible memberpreventing rotation of said platen about said axis of said post memberand transmitting forces between said platen and said post member indirections parallel to said face of said platen; and a pivoting membermounted between said platen and said post member; wherein said flexiblemember and said pivoting member maintain a substantially constantpositioning of said platen with respect to said post member indirections perpendicular to said axis of said post member, whileallowing pivoting of said platen about said pivoting member, andmovement between said post member and said platen along a direction ofsaid axis of said post member.
 5. The apparatus of claim 4, wherein saidflexible member substantially prevents rotation of said platen in aplane perpendicular to said axis of said post member.
 6. The apparatusof claim 4, wherein said flexible member comprises an annular flexiblering mounted around a perimeter of said platen and to an inner perimeterof said post member.
 7. The apparatus of claim 6, wherein said flexiblering is made of thin steel.
 8. The apparatus of claim 4, wherein saidpivoting member comprises a guide ball.
 9. The apparatus of claim 8,further comprising:a guide ball socket formed in said post member, saidguide ball socket mating with said guide ball and substantiallypreventing movement of said guide ball in directions perpendicular tosaid axis of said post member, while allowing movement of said guideball in directions aligned with said axis of said post member and alsoallowing pivoting of said guide ball.
 10. The apparatus of claim 9,wherein said guide ball is securely fastened to said platen.
 11. Theapparatus of claim 4, further comprising:at least one biasing memberbetween said post member and said platen, said biasing member biasingsaid post member and said platen away from one another.
 12. Theapparatus of claim 11, further comprising:an annular notch formed insaid post member, said annular notch receiving said at least one biasingmember.
 13. The apparatus of claim 12, wherein said at least one biasingmember comprises at least one compression spring.
 14. The apparatus ofclaim 13, wherein said at least one compression spring comprises a stackof circular Bellville springs that are received in said annular notchand abut said platen.
 15. The apparatus of claim 4, further comprising:atable having an upper surface and a central axis perpendicular to saidupper surface; and a polishing medium attached to said upper surface ofsaid table; wherein said post member and said platen are adapted toforce the substrate against said polishing medium and said upper surfaceduring polishing.
 16. The apparatus of claim 15, furthercomprising:means for preventing rotation of said post member and saidplaten about said central axis of said table during polishing.
 17. Theapparatus of claim 16, further comprising:means for providing relativemovement between said platen and said upper surface of said table. 18.The apparatus of claim 17, wherein said means for providing relativemovement is capable of providing movement between said platen and saidupper surface of said table in any direction within a plane of saidupper surface of said table.